High-pressure Spark Plasma Sintering Research Articles

Comparisons between the high-pressure SPS and routine SPS of dense YH2-x

Spark plasma sintering (SPS) shows great potential in the fabrication of bulk YH2-x. For the first time, the high-pressure SPS (HPSPS) of YH2-x is explored and compared with the routine SPS. It is found that HPSPS remarkably reduces the densification temperature, and thus the thermal dehydrogenation phase transition (δ-YH2-x → α-Y) and the hydrogen loss of YH2-x can be avoided. Nearly fully dense δ-YH1.80 monolith is successfully prepared by HPSPS under 500 MPa at a relatively low temperature of 800 °C. HPSPS is effective in overcoming the thermal dehydrogenation problem during the sintering of YH2-x. Compared with significant grain growth in the routine SPS, the grain growth in the HPSPS is very limited. In terms of thermal properties, the thermal conductivity of HPSPS prepared YH2-x is relatively low below 600 °C, then tends to be closed above 600 °C, compared with routine SPS prepared and directly hydrided YH2-x. The thermal expansion of HPSPS prepared dense YH2-x is basically consistent with that of directly hydrided bulk. This study provides a valuable reference for the sintering of high-quality δ-YH2-x and other metal hydrides monoliths.

Ultra-fast preparation of UO2 fuel pellets by high pressure spark plasma sintering

A new sintering technique as a combination of ultra-fast spark plasma sintering and high-pressure sintering was employed to densify UO2 fuel pellets. UO2 pellets with a relative density of 97.5 % and inhibited grain growth were fabricated at 1000 °C under 150 MPa within only 4.5 min. Both short sintering time and high pressure might explain the inhibited grain growth and hence finer microstructures. UO2 fuel pellets prepared in this work exhibit excellent mechanical properties.

Densification of zirconia-based ceramics by non-conventional sintering processes and chemical pathways

The present work reports an overview of the main results we have obtained from non-conventional sintering approaches towards the densification of 3mol.% yttria stabilized zirconia (3YSZ), the lessons learned from these results, and open questions that still need to be addressed. A wide range of techniques have been developed in the field of ceramic sintering, both in high temperature regime with extreme heating rates (>10 000 °C/min), as well as in very low temperatures (<400 °C) regime involving chemical reactivity to activate sintering mechanisms. In this context, we have recently explored several of these methods in order to understand and control the different sintering parameters and their impact on microstructure, crystallinity, grain boundaries and defect chemistry. Due to its complex chemistry, 3YSZ is a playground for the exploration of sintering capabilities and their influence on ceramic properties. Our recent studies in this field are presented in the present work, highlighting the effect of high heating rates through Flash Spark Plasma Sintering ( Flash-SPS) and Ultra High temperature Sintering (UHS) on sintering, but also the combination of low temperature Cold Sintering Process (CSP) and the associated precursors chemistry, with Spark Plasma Sintering (SPS). It also shows the possible densification by reactive hydrothermal sintering, using hydroxide precursors, and the effect of High Pressure SPS (HP-SPS) on 3YSZ nanopowders. All of these results touch upon current trends in the field of sintering, and highlight the possibilities in terms of chemical reactivity of precursors and physical parameter control. In particular, they show the importance of defect chemistry, related to the sintering atmospheres and their impact on both microstructures and properties.

Densification, hydrogen retention, microstructure, and mechanical properties of ε-ZrH2-x monolith fabricated by high-pressure spark plasma sintering

The sintering of zirconium hydride (ZrH2-x) without hydrogen atmosphere faces the challenge of thermal dehydrogenation problem, which is overcome by the high-pressure spark plasma sintering (HPSPS) technique in this study. The application of high pressure remarkably reduces the initial densification temperature and effectively restrains the thermal dehydrogenation of ZrH2-x in SPS. Under 500 MPa loading, ε-ZrH1.72 monolith with high densification (96.62 %) and high hydrogen density (6.04 × 10 22 atoms H/cm3) is sintered at a relatively low temperature of 700 °C for only 5 min. Meanwhile, the grain growth of ZrH2-x is very limited. The Young’s modulus and hardness of the sintered ε-ZrH1.72 are 58.68 GPa and 2.61 GPa respectively. Compared with the directly hydrided ε-ZrH2-x, the sintered ε-ZrH1.72 has a more uniform hardness distribution and improved fracture toughness of 3.19 MPa m1/2. This study provides a valuable reference for the sintering of other easily decomposable materials.

Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3

Sm2Fe17N3 powders were prepared by a novel molten-salt mechanochemically assisted reduction-diffusion (RD) approach. CaCl2, plays a critical role during mechanochemical processing as a dispersant, facilitating the RD process by dissolving the precursors in a molten flux and further assisting during the washing step by efficient removal of by-products thus minimizing the surface oxidation of Sm2Fe17N3 powder particles. Various milling times and RD temperatures were examined, and synthesis conditions were optimized to achieve pure-phase Sm2Fe17N3 powders with low aggregation of magnetic particles. Powders synthesized at 950 °C RD exhibited the highest hard-magnetic properties: coercivity Hc of 13.5 kOe, and maximum energy product (BH)max of 19.4 MGOe. This was attributed to the formation of fine single-phase Sm2Fe17N3 particles as well as minimal oxidation of particle surface. Furthermore, by densifying the Sm2Fe17N3 powders with high-pressure spark plasma sintering, a bulk magnet with (BH)max of 16.5 MGOe and a relative density of 86 % was produced indicating that the obtained Sm2Fe17N3 powders had low oxygen content.

Powder metallurgy process enables production of high-strength conductive Cu-based composites reinforced by Cu50Zr43Al7 metallic glass

This study showcases the preparation of Cu50Zr43Al7 metallic glass-reinforced ultrafine grain copper matrix composites using a powder metallurgical process to achieve a superior combination of strength and electrical conductivity. The mechanical ball milling (BM) process is utilized to effectively refine the Cu matrix grains to the nanoscale. Furthermore, high-pressure spark plasma sintering (SPS) is employed to prevent grain growth and improve the interface bonding between two phases. Increasing the Cu50Zr43Al7 addition content improved the mechanical properties under a load transfer strengthening strategy. Notably, the 20 wt% Cu50Zr43Al7/Cu composite demonstrates excellent mechanical properties and high electrical conductivity. This is attributed to the uniformly-dispersed Cu50Zr43Al7 metallic glass particles in the ultrafine grain Cu matrix with well-established interfacial bonding. The synergistic strengthening of the Cu50Zr43Al7/Cu composites represents a novel choice for switch contact materials application, maintaining excellent mechanical properties while satisfying conductive requirements.

Review on high-pressure spark plasma sintering and simulation of the impact of die/punch material combinations on the sample temperature homogeneity

This paper provides a thorough review on high-pressure spark plasma sintering (HP-SPS) experiments conducted to date, with a focus on different tooling materials and their impact on microstructural, mechanical and optical properties of the resulting materials. Any SPS experiment conducted with a pressure over 200 MPa was considered as “high-pressure”. This review was supported by a comprehensive finite-element modelling study of the temperature distribution in the sample and tool set-up as a function of the used geometries, tooling material combinations, temperature window and sample material properties. The effect of the electrical and thermal conductivities of the tool and sample material on the temperature gradients in some specific sample-tool material combinations was elucidated. Electrically and thermally conductive samples in combination with SiC tools showed the lowest temperature gradients during processing, whereas electrical insulators in combination with high pressure WC, steel or TZM tools with an inner graphite die showed the smallest temperature gradients over the sample.

Effect of Mn doping on the densification and properties of transparent alumina by high-pressure spark plasma sintering

It is common practice to control the densification and microstructure of alumina using additives or dopants. In the present study, the effect of Mn-doping on the sintering behavior, optical properties, microstructure, and mechanical properties of alumina was examined. Addition of 0.5 at.% Mn was shown to significantly reduce densification rate and grain growth during high-pressure spark plasma sintering (HPSPS). The major effects resulted from the formation of MnAl2O4 nano inclusions located at grain boundary triple points. The Mn-doped alumina was highly transparent and had a light-brown tint due to absorption between ∼410 and 580 nm and a cutoff at 300 nm. The MnAl2O4 nano-inclusions caused the Young's modulus to decrease from 400 ± 5 GPa for pure alumina to 370 ± 5 GPa for Mn-doped alumina, while the fracture toughness increased from 2.97 to 3.66 MPa m0.5, and the hardness was the same. Thus, the addition of Mn enables control of the transparent alumina densification process and microstructure development, and it can be used to tailor the optical and mechanical properties for specific applications.

Densification mechanism, microstructure and mechanical properties of ZrC ceramics prepared by high-pressure spark plasma sintering

The traditional way of densifying high-melting-point ceramics at high temperatures with long soaking time leads to severe grain coarsening, which degrades the mechanical properties of ceramics. Here, highly dense (∼98%) zirconium carbide (ZrC) ceramics with limited grain growth were obtained by spark plasma sintering (SPS) at relatively low temperatures, 1900 ℃, with a high pressure up to 200 MPa in a reliable carbon-fiber-reinforced carbon composite (Cf/C) mold. Subgrains and high-density dislocations formed in the high-pressure sintered ceramics. The hardness and fracture toughness of the prepared highly dense ZrC ceramics reached 20.53 GPa and 2.70 MPa·m1/2, respectively. The densification mechanism was mainly plastic deformation under high pressure. In addition, ZrC ceramics sintered at high pressure possessed a high dislocation density of 7.30 × 1012 m−2, which was suggested to contribute to the high hardness.

Recent Developments of High-Pressure Spark Plasma Sintering: An Overview of Current Applications, Challenges and Future Directions.

Spark plasma sintering (SPS), also called pulsed electric current sintering (PECS) or field-assisted sintering technique (FAST) is a technique for sintering powder under moderate uniaxial pressure (max. 0.15 GPa) and high temperature (up to 2500 °C). It has been widely used over the last few years as it can achieve full densification of ceramic or metal powders with lower sintering temperature and shorter processing time compared to conventional processes, opening up new possibilities for nanomaterials densification. More recently, new frontiers of opportunities are emerging by coupling SPS with high pressure (up to ~10 GPa). A vast exciting field of academic research is now using high-pressure SPS (HP-SPS) in order to play with various parameters of sintering, like grain growth, structural stability and chemical reactivity, allowing the full densification of metastable or hard-to-sinter materials. This review summarizes the various benefits of HP-SPS for the sintering of many classes of advanced functional materials. It presents the latest research findings on various HP-SPS technologies with particular emphasis on their associated metrologies and their main outstanding results obtained. Finally, in the last section, this review lists some perspectives regarding the current challenges and future directions in which the HP-SPS field may have great breakthroughs in the coming years.

Impact response of germanium over 300–1143 K temperature range

Impact response of &amp;lt;111&amp;gt; oriented germanium single crystals and polycrystalline samples obtained by high-pressure spark plasma sintering of pure germanium powder was studied in two series of planar impact tests performed at 300 and 1143 K with samples of different thicknesses and in a series of tests with 2 mm single crystals preheated up to the temperatures 300–1143 K. In all the tests, the samples were shock-loaded by tungsten impactors having velocity 980 ± 40 m/s, while the velocity of the interface between the germanium sample and the fused silica window was continuously monitored by velocity interferometer. Under compression, the cubic diamond (cd) germanium transforms into its high-pressure (β-Sn or liquid) modification. The stress corresponding to the upper bound of the existence of impact loaded cd germanium was found to depart upward from that obtained in the static experiments. At temperatures greater than 900 K, this departure increases due to the initiation of melting in the shock-loaded material. Part of the velocity histories recorded with either single or polycrystalline samples was characterized by a four-wave (instead of the expected three-wave) structure. This “surplus” wave seems to be caused by a short-term existence of an intermediate (nonequilibrium) germanium phase which, however, does not affect the principal germanium Hugoniot.

Effect of synthesis route on optical properties of Cr:Al2O3 transparent ceramics sintered under high pressure

Transparent polycrystalline ceramics doped with active ions are suitable for many potential optical applications. Fabrication methods that utilize applied external pressure show promise as they allow reducing sintering temperatures and produce transparent non-cubic ceramics. However, not much attention has been given to the dissolution of doping elements while sintering under relatively high pressure and low temperature conditions. In this study, we employed high-pressure spark plasma sintering (HPSPS) for fabrication of Cr:Al 2 O 3 ceramics and investigated the effect of two different synthesis routes on densification and optical properties. Namely, HPSPS of Cr-doped alumina powders prepared by directly doping with 0.05, 0.2, and 0.5 Cr at.% via co-precipitation and alumina mixed with similar mol.% of Cr 2 O 3 nanoparticles towards doping via solid-state reactive sintering (so-called ‘synthesized’ and ‘mixed’, respectively). Remarkable differences in optical properties between samples obtained by each method were observed. The ceramics fabricated from synthesized powders exhibited pink shades of ruby, high transparency, and strong photoluminescence. In contrast, the mixed ceramics exhibited green color, low transparency, and weak photoluminescence. It was found that in the green samples, most of the Cr 2 O 3 nanoparticles remain as undissolved nano-inclusions. These inclusions are detrimental to optical properties and cause green pigmentation of the alumina. Post-sintering heat treatment dissolved the inclusions and the added Cr 3+ in alumina increased photoluminescence. Evidently, doping transparent alumina via reactive sintering is not viable for processes such as HPSPS. Nevertheless, rapid low temperature densification might enable the design of functional translucent ceramics with integrated second phase nanoparticles. • High-pressure spark plasma sintering was used to fabricate polycrystalline Cr:Al 2 O 3 . • Cr-doping via co-precipitation yielded highly transparent Cr 3+ :Al 2 O 3 ceramics. • Mixed Cr 2 O 3 -Al 2 O 3 yielded green translucent ceramics due to Cr 2 O 3 nano-inclusions. • Post-sintering heat treatment dissolved Cr 2 O 3 and increased photoluminescence. • Reactive sintering is not a viable synthesis route for doped transparent alumina.

High Pressure (HP) in Spark Plasma Sintering (SPS) Processes: Application to the Polycrystalline Diamond.

High-Pressure (HP) technology allows new possibilities of processing by Spark Plasma Synthesis (SPS). This process is mainly involved in the sintering process and for bonding, growing and reaction. High-Pressure tools combined with SPS is applied for processing polycrystalline diamond without binder (binderless PCD) in this current work. Our described innovative Ultra High Pressure Spark Plasma Sintering (UHP-SPS) equipment shows the combination of our high-pressure apparatus (Belt-type) with conventional pulse electric current generator (Fuji). Our UHP-SPS equipment allows the processing up to 6 GPa, higher pressure than HP-SPS equipment, based on a conventional SPS equipment in which a non-graphite mold (metals, ceramics, composite and hybrid) with better mechanical properties (capable of 1 GPa) than graphite. The equipment of UHP-SPS and HP-SPS elements (pistons + die) conductivity of the non-graphite mold define a Hot-Pressing process. This study presents the results showing the ability of sintering diamond powder without additives at 4–5 GPa and 1300–1400 °C for duration between 5 and 30 min. Our described UHP-SPS innovative cell design allows the consolidation of diamond particles validated by the formation of grain boundaries on two different grain size powders, i.e., 0.75–1.25 μm and 8–12 μm. The phenomena explanation is proposed by comparison with the High Pressure High Temperature (HP-HT) (Belt, toroidal-Bridgman, multi-anvils (cubic)) process conventionally used for processing binderless polycrystalline diamond (binderless PCD). It is shown that using UHP-SPS, binderless diamond can be sintered at very unexpected P-T conditions, typically ~10 GPa and 500–1000 °C lower in typical HP-HT setups. This makes UHP-SPS a promising tool for the sintering of other high-pressure materials at non-equilibrium conditions and a potential industrial transfer with low environmental fingerprints could be considered.

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co2+:MgAl2O4 saturable absorbers for laser passive Q-switching. Densification of commercial MgAl2O4 powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co9S8 inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co2+:MgAl2O4. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co2+, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σgs) and excited (σes) cross-sections of 3.5×10-19 and 0.8×10-19 cm2, respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co2+:MgAl2O4 ceramics.

Interface configuration effect on mechanical and tribological properties of three-dimension network architectural titanium alloy matrix nanocomposites

Strong interfacial bonding is the primarily crucial issue in titanium matrix composites (TMCs). In this study, three-dimensional (3D) network structured Ti6Al4V (TC4) matrix TMCs with 0.15 wt% few-layer graphene (FLG) were fabricated by high pressure spark plasma sintering (SPS, 200–300 MPa) at 850 ℃, 900 ℃ and 950 ℃, correspondingly producing three types of interface configuration with in-situ TiC to FLG ratio of 4:6, 7:3 and 9:1. Composites sintered under 900 ℃ displayed outstanding integrated properties, including high micro-hardness (345.2 HV), excellent ultimate tensile strength (UTS, 1041 MPa) with acceptable tensile ductility (ε, 9.3%) and wear resistance with the lower wear rate (ω, 5.3 × 10−4 mm3/Nm). The synergistic effect of moderately generated TiC reinforcements on FLG (ratio of 7:3) in 3D network boundary under 900 ℃ leads to efficient load-bearing capacity, acting as the dominant strengthening mechanism. Self-lubricating effect of FLG and debris strengthening of TiC and TiO2 are also responsible for improved wear resistance.

Ultra-Fine Grained Tungsten Heavy Alloy Prepared by High-Pressure Spark Plasma Sintering

Ultra-Fine Grained Tungsten Heavy Alloy Prepared by High-Pressure Spark Plasma Sintering

Synthesis and behavior of bulk iron nitride soft magnets via high-pressure spark plasma sintering

In this study, dense bulk iron nitrides (FexN) were synthesized for the first time ever using spark plasma sintering (SPS) of FexN powders. The Fe4N phase of iron nitride in particular has significant potential to serve as a new soft magnetic material in both transformer and inductor cores and electrical machines. The density of SPSed FexN increased with SPS temperature and pressure. The microstructure of the consolidated bulk FexN was characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRD revealed a primary phase of Fe4N with secondary phases of Fe3N and metallic iron. Finite element analysis (FEA) was also applied to investigate and explain localized heating and temperature distribution during SPS. The effects of processing on interface bonding formation and phase evolution were investigated and discussed in detail to provide insight into fundamental phenomena and microstructural evolution in SPSed FexN.Graphic abstract

Al excess extends Hall‐Petch relation in nanocrystalline zinc aluminate

AbstractThe increase in hardness with decreasing grain size is a well‐established size effect known as the Hall‐Petch relationship. In ceramics, there has been controversy surrounding the existence of a low size limit below which size‐induced hardening no longer occurs and softening is observed instead. Here, this so‐called inverse Hall‐Petch relationship was observed in quasi‐stoichiometric dense nanocrystalline zinc aluminate while an extension of the normal Hall‐Petch behavior was demonstrated by Al‐rich zinc aluminate nanoceramics. Vickers hardness increased with grain refinement for quasi‐stoichiometric samples prepared by High Pressure Spark Plasma Sintering, exhibiting a maximum of 18.6 GPa at a grain size of 21.4 nm. Conversely, Al‐rich zinc aluminate produced by the same technique strengthened up to 19.2 GPa at 12.6 nm grain sizes. Cross‐sections of Vickers indentation imprints showed that while quasi‐stoichiometric zinc aluminate showed a change in sub‐surface cracking pattern from larger to smaller grain sizes (before and after the inverse Hall‐Petch), the Al‐rich samples had sub‐surface cracking similar to those found in large grain sizes in quasi‐stoichiometric samples. These results suggest that softening at small grain sizes is driven by the activation of shear and fracture at weak grain boundaries which can be mitigated by Al enrichment.

Single-step, high pressure, and two-step spark plasma sintering of UO2 nanopowders

Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650−550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO2 samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO2.00.

Optical properties of transparent polycrystalline ruby (Cr:Al2O3) fabricated by high-pressure spark plasma sintering

Doped transparent ceramics with high optical quality can serve as materials for photonic applications such as laser gain media. In that regard, transparent polycrystalline alumina has potential for high-power applications due to its excellent physical and chemical properties, combined with unique doping possibilities. However, optical birefringence of Al2O3 crystals make achieving sufficiently high optical transmittance a processing challenge. In the present study, we demonstrated fabrication of highly transparent 0.5 at.% Cr:Al2O3 ceramics by high-pressure spark plasma sintering (HPSPS). The optical properties of these polycrystalline ruby ceramics were analyzed in order to assess possible laser operation (at 694.3 nm). The obtained ceramics exhibit high in-line transmittance (∼72.5 % at 700 nm), equivalent to a scattering coefficient of 2.15 cm−1, and characteristic ruby photoluminescence. The theoretically estimated lasing threshold and percentage of absorbed pump power indicate that such ruby ceramic lasers could operate at reasonable thresholds of 80−225 mW with short lengths of 0.5−5 mm. Thus, HPSPS is a promising method for producing laser-quality doped transparent ceramics for compact laser systems.